1. Field of the Invention
The present invention relates to a polishing apparatus for polishing the surface of a plate-shaped object. The invention utilizes relative movement between the object and a polishing cloth, while pressing the two together. More particularly, the invention is directed to a polishing apparatus for polishing silicon wafers, or other thin semiconductor disks, with high accuracy.
2. Description of the Related Art
Recently, there has been remarkable progress in miniaturization and high integration of semiconductor devices. The technological progress has led to the current age of VLSI circuits having more than 100,000 devices per chip. With this progress, development of the optolithographic technology for drawing an IC pattern on a thin semiconductor plate has rapidly been promoted to such a level that it can draw an IC pattern having a much narrower line width: a 1-.mu.m line width for a 1M-bit dynamic RAM and a 0.8-.mu.m line width for a 4M-bit dynamic RAM, for example. Such a technology is employed in, for example, a laser stepper apparatus of a projection exposure type.
Such an exposure optical system needs a greater number of apertures in order to realize the IC patterns having a very narrow line width. Therefore, it is inevitable to decrease the depth of the focus. This requires a higher accuracy in the flatness of the surface of a thin semiconductor disk, onto which an IC pattern is projected.
An example of a polishing apparatus for such a thin semiconductor disk is illustrated in FIG. 9. This polishing apparatus comprises a turn table (hereinafter referred to as a surface table 2) having a polishing cloth 1 stuck on its top surface and being rotatable by an external driving force, a plate 91 disposed above the polishing cloth-stuck surface (hereinafter referred to as a polishing surface 1A) and having one or more thin semiconductor plates 3 adhered or stuck to its bottom surface, and a mount head (hereinafter referred to as a head section 92) for applying a pressurizing force from the top of the plate 91 by using a pressing shaft 93. The polishing apparatus causes a polishing or rubbing movement between the underside 95 of each thin semiconductor disk 3 and the polishing cloth 1 (polishing material) while dispersing the polishing agent (wet type or dry type) containing abrasive grains, such as SiO.sub.2 or Fe.sub.2 O.sub.3, on the polishing cloth 1 through a polishing agent dispersion unit 94 or the like, whereby the surface of the thin semiconductor disk 3 is polished at a high accuracy based on a so-called mechanochemical polishing method (a combination of mechanical polishing and chemical polishing). This polishing apparatus will be hereinafter referred to as the first prior art.
According to the above apparatus, however, due to frictional resistance between the underside 95 of each thin semiconductor disk 3 and the polishing cloth 1, the plate 91 having the thin disks 3 secured thereto is tilted downward at its leading edge and causes a relative increase in pressurizing force on the leading edge of the disk 3 from the polishing cloth 1. Consequently, even if the leading edge of the plate 91 is shifted along the periphery, resulting from compulsory or natural rotations about its own axis, while the plate 91 is pressed on the rotational surface table 2, so that the plate 91 rotates in a relative planetary motions with the rotational surface table 2, the superficial stock removal of thin semiconductor disks 3 does not become uniform over each wafer so that high flatness of the surface of the polished thin plates 3 cannot be realized.
As a solution to this shortcoming, there was proposed a structure as shown in FIG. 8. As illustrated in this diagram, a hollow section 96 defined between a head section 82 and a partition film 89 is filled with a fluid 88 so that the partition film 89 comes in close contact with the top of a plate 81. A ring-shaped retainer 87 is fit in a ring-shaped gap formed around the plate 81 to securely support the peripheral portion of the partition film 89. This structure can restrict movement between the plate 81 and head section 82 in a plane parallel to the polishing surface 1A. This polishing apparatus, which is disclosed in, for example, Published Unexamined Japanese Patent Application No. 63-52967, will be hereinafter referred to as the second prior art.
According to the second prior art, it may appear possible to overcome the problem identified in the first prior art. Since, in theory, the fluid 88 evenly presses the top of the plate 81 through the partition film 89, a pressure applied between the thin semiconductor disks or wafers 3 and the polishing cloth 1 becomes uniform. On the contrary, the solution cannot be realized for the following reason. During polishing, a frictional force S generated between each wafer 3 and the polishing cloth 1 acts on the plate 81. This force S acts in the rotational direction of the surface table 2 on the polishing surface 1A. There also exists a reaction force R against the frictional force, which acts on the plate 81 from the retainer 87 at the contact portion therebetween. No consideration whatsoever is paid to the direction of action of this reaction force R acting in a plane parallel to the polishing surface 1A from a structural point of view.
Since the frictional force S and reaction force R do not exist in the same plane, a third force originating from the contacting pressure between the polishing cloth and wafers should be involved in order to balance the force acting on the plate 81. According to the second prior art, however, since the direction of action of the reaction force R is parallel to the polishing surface 1A, a rotation moment is generated on the plate 81 to tilt the plate 81. This produces non-uniform contacting pressure or pressurizing force between the polishing cloth 1 and wafers 3, so that the wafer surface cannot be polished with a high flatness.
Both of the prior art apparatuses are so designed that a plurality of thin semiconductor plates 3 are secured to the bottom of a single plate 81 or 91. Due to an unavoidable slight variation in thickness of the thin disks 3, the parallelism between the plate 81 or 91 and the polishing surface 1A may not be maintained at high accuracy. Accordingly, slight tilting of the plate 81 or 91 is likely to result in non-uniform pressure acting on the thin disks 3, so that high flatness of the polished surface of each thin semiconductor plate 3 cannot be realized.
As a solution to this shortcoming, the applicant previously proposed a structure as shown in FIG. 7. As illustrated in this diagram, a single thin disk 3 is secured to a plate 71 and this plate 71 is supported by a so-called spherical bearing 79 disposed between the plate 71 and a head section 72. That portion of the spherical bearing 79 which is on the side of the top of the plate 71 is shaped to have a convex surface 71a and that portion of the bearing 79 which is on the side of the bottom of the head section 72 is shaped to have a concave surface 72a. The operational center (the center of the supporting force P) of the plate 71 or the center P of the spherical bearing 79 is located on the polishing surface 1A. This polishing apparatus, which is disclosed in, for example, Published Unexamined Japanese Patent Application No. 63-62668, will be hereinafter referred to as the third prior art.
According to the third prior art, the operational center P of the pressing force coincides with the polishing surface 1A and uniform load can be applied to the plate 71 by the spherical bearing 79. Therefore, the force S from the polishing cloth originating from the aforementioned frictional resistance acts in the same plane where the operational center of the pressing force exists. This should prevent the plate 71 from tilting and can produce substantially uniform polishing pressure on the underside of the thin disk 3 secured to the underside of the plate 71, thus ensuring surface polishing at a high flatness.
The structure of the mechanical supporting means of the spherical bearing 79 wherein the convex surface 71a and the concave surface 72a are in surface contact with each other and the structure wherein a pressure is applied on the plate 71 by way of the pressing shaft 73 unavoidably cause the frictional resistance between the plate 71 and the head section 72 to be great. As a result, even if a slight displacement of the surface table 2 occurs while the surface table 2 is rotating, the plate 71 cannot closely follow the surface table 2, thus making it difficult to perform surface polishing with a high flatness.